The production of high quality articles, particularly electronic, tape, optical, photographic, photothermographic, thermographic, abrasives, adhesive, display, and pharmaceutical articles, oftentimes depends on the ability to reliably and uniformly apply a thin film of a coating solution onto a continuously moving substrate or web. Thin films can be applied using a variety of techniques, including dip coating, forward and reverse roll coating, wire wound rod coating, and die type coating. Die coaters include knife coaters, slot coaters, slide coaters, slide curtain coaters, drop die curtain coaters, and extrusion coaters among others. Many types of die coaters are described in the literature, such as by Edward Cohen and Edgar Gutoff, Modern Coating and Drying Technology (VCH Publishers: NY 1992, ISBN 3-527-28246-7), and Gutoff and Cohen, Coating and Drying Defects Troubleshooting Operating Problems (Wiley Interscience, NY, ISBN 0-471-59810-0), and Liquid Film Coating, (Chapman and Hall, Eds, NY, 1997, ISBN 0-142-06481-2
Die coating is a process whereby a pressurized stream of coating material is moved through an internal cavity or reservoir of a die coater and discharged from an applicator slot to form a ribbon of coating material. As mentioned previously, it is important that the applied coating, i.e., the ribbon, exhibit uniformity. By “uniformity,” it is meant that the applied coating should measure substantially the same thickness throughout the ribbon. Uniformity of the applied coating is especially important for products used in critical optical or electronic applications. (Coatings can be applied as a single layer or as two or more superimposed layers upon a substrate. Although it is usually most convenient for the substrate to be in the form of a continuous web, a substrate may also be formed as a succession of discrete sheets.) The uniformity of the coating layer is influenced by the precision of the geometry of the coating slot through which the coating layer (or extrudate) passes.
All die coaters have at least one slot. Each slot has a slot length corresponding to the cross-sectional width of the applied coating, a slot width corresponding to the distance from the internal cavity to the exit of the slot, and a slot height, which is the narrow dimension of the slot between the two parallel surfaces defining the slot, itself. One of the fundamental problems associated with die type coaters is the ability to accomplish a uniform flow per unit width across the entire length of the slot of the die. A fundamental issue in attaining this uniformity of the flow, and thus the critical uniformity of the applied coating, is the ability to construct a die with the best possible precision of the die slot “height” (e.g., a uniform die slot “height” across the entire width of the slot).
It is desirable to construct an applicator slot that exhibits a constant height across the entire length of the slot. Any variation in the slot die height degrades the uniformity of the applied coating discharged therefrom. The uniformity of an applicator slot height is a function of, amongst other things, the geometry of the two halves of the die forming the applicator slot. It is therefore desirable to understand the influence of geometric choices concerning the two halves of the die upon the expected uniformity of the applicator slot height.